This invention relates to reassembly of data packets into messages, in an Asynchronous Transfer Mode communications system.
Asynchronous transfer mode (ATM) network interfaces are becoming popular, especially in networks which support data transfer rates in excess of 1.544 Mbits per second in North America and 2.0 Mbits per second in Europe. It is well known that the asynchronous mode of transmitting and switching information in a network provides the advantages of mixing data, voice and video on the same medium.
In an ATM transmission, a transmitting network node represents a message as a plurality of data packets. Different messages may have different lengths and, therefore, different numbers of data packets. For example, a carriage return message would have a smaller length than a message representing a frame of video.
Each data packet includes 53 octets or bytes, 48 of which represent data and the remaining 5 of which represent control information including a message identification and an end of message status. These 53 octets are transmitted over an ATM network by a Physical Layer Interface (PHY) which receives signals from a host computer via an interface known as a Universal Test and Operations Physical Layer Interface for ATM (UTOPIA) and which creates the physical signals used on the network to effect data and handshaking functions. The physical signals may include optical signals for transmission over a fibre optic cable.
At a receiving node on the ATM network, a similar physical layer interface receives physical signals from the above and other transmitters and produces system-level physical signals in the UTOPIA format. Such signals include data signals which represent the 53-octet ATM data packets originally transmitted from each transmitter. However, it is possible, and in a busy network probable, that a 53-octet ATM data packet from another transmitter may be received before a next data packet from the original transmitter is received. Thus, the receiver receives a succession of data packets where each data packet may be from a different transmitter. The receiver then has the task of re-assembling into a contiguous sequence, all data packets of each respective message.
One method of data packet reassembly is disclosed in a data sheet for a 155 Mb/s ATM Segmentation & Reassembly "SAR" Controller for the PCI local Bus, published October 1995 by Integrated Device Technology, Inc. of Santa Clara, Calif., USA. This method involves the use of a relatively small free buffer, of a fixed length, typically less than 128 octets, to receive at least initial cells of a given data packet. If the message includes a large number of data packets, the small free buffer can become full in which case, relatively large free buffers, of a fixed length, typically 4K octets, are appended to the small free buffer to accommodate the remainder of the packets in the message. However, memory is wasted in this approach because, for example, since each data packet includes 53 octets, 3 data packets will require the use of one small buffer and one large buffer which will cause approximately 3K of the large 4 buffer to be left unused. This can be wasteful of memory, particularly where the receiving node is expected to received a large number of data packets on a large number of channels. The present invention addresses these problems.